COMMENTARY
Disrupted Vasculature and Blood–Brain Barrier in Huntington Disease
H
untington disease (HD) is characterized by neuronal loss in the caudate nucleus and putamen as well as heterogeneous patterns of neurodegeneration throughout various regions of the cerebral cortex and in many other regions of the brain. Two new studies by Hsaio et al1 and Drouin-Ouellet et al2 report that the cerebral vascular system is also highly disrupted in HD, adding a new dimension to the horizons of pathological mechanisms operating in this tragic monogenetic disease. The methods used to detect these changes included scanning HD-affected subjects using magnetic resonance imaging (MRI) as well as morphological studies of human postmortem brain tissue and brain tissue of animal models of the disease. The disruption of the vasculature and blood–brain barrier (BBB) has been strongly implicated in other neurodegenerative diseases such as Alzheimer disease3 and Parkinson disease.4 In the case of Alzheimer disease, there is deposition of b-amyloid in arteries resulting in cerebral amyloid angiopathy, which is present in >80% of Alzheimer disease cases.5 This causes dysfunction of vascular integrity, leading to leakage through the BBB allowing normally highly regulated molecules to cross into the brain, which can cause inflammatory cascades and release neurotoxins into the intracellular spaces of the brain. In addition, loss of vascular integrity leads to loss of blood flow and subsequent hypoxic regions throughout the brain as well as a lack of clearance of neurodegenerative products. The recognition of these types of vascular changes is only now becoming evident in HD. A previous study6 reported increased blood vessel density but not blood vessel size in the R6/2 mouse model of HD, as well as increased vessel density in the caudate nucleus of postmortem HD human brain. The results published in this new report by Hsiao et al1 further advance the field by showing marked changes in MRI that are measured as vascular reactivity in different mouse models of HD. The results of these knockin models indicate that the vascular density changes occur when mutant protein is expressed in both neurons and astrocytes rather than neurons alone. Additionally, there is altered cerebral blood flow in these mouse models that mimics the changes observed in human brains. One of the mechanisms highlighted in these studies is that an inflammatory response may lead to vessel changes. This was proposed by observing higher than normal levels of vascular endothelial growth factor A in astrocytes in brains C 2015 American Neurological Association 158 V
of mouse models and in human HD brain and that this promotes proliferation of new blood vessels, leading to increased vessel density. This inflammatory response was enhanced through an IjB kinase-nuclear factor jB– dependent pathway, and it may be through this mechanism that proliferation of endothelial cells is triggered; also, this importantly leads to the reduced coverage of pericytes. The second study by Drouin-Ouellet et al2 revealed that in HD patients there is a significant increase in cerebral blood flow in cortical gray matter but not in caudate or putamen, although the results did not yield differences in blood vessel densities in these scans. Following on from these imaging studies, morphometric studies of human postmortem brain found that in the putamen of HD patients there was an increased density of small blood vessels with a larger proportion of small blood vessels compared to medium-sized blood vessels; when correlated with putaminal surface area, this suggested that putaminal atrophy is accompanied by a reduction in blood vessel size. These changes in human brain also correlated with changes observed in the striatum of the R6/2 mouse, where the density of blood vessels was increased and there was a reduction in blood vessel diameter. Investigations at the cellular level in the human HD brain found inclusions in about 3 to 7% of cortical blood vessels that were apparent in all compartments of the neurovascular unit, including the basement membrane, endothelial cells, perivascular macrophages, and smooth muscle actin (SMA)-positive pericytes. In the R6/2 mouse, inclusions were found in the basal membrane, a-SMA positive pericytes or smooth muscle cells, and endothelial cells. There was also evidence of leakage of the BBB in the putamen of HD brains and in the striatum of the R6/2 mice. There was a decrease in proteins associated with tight junctions and an increase in vesicles associated with transcytosis, which may together increase BBB permeability. Recent studies suggest that pericytes regulate the maintenance of the BBB.7 In Alzheimer disease and in motor neuron disease, reduced capillary pericyte coverage has been associated with BBB leakage.8,9 Although Drouin-Ouellet et al2 did not find reduced numbers of pericytes in R6/2 mice using transmission electron microscopy, in the study by Hsaio et al pericyte coverage was found to be reduced in both the human HD brain
Waldvogel et al.: Disrupted Vasculature in HD
and in R6/2 mice using desmin or platelet-derived growth factor receptor b to label pericytes. Hsaio et al further showed that astrocytes may be responsible for pericyte loss in HD. Thus, further study of pericytes in HD and its models is vital in understanding vascular dysfunction in this disease. Because pericytes are critically involved in maintaining the BBB, compounds that augment their proliferation and/or viability may prove useful in helping to maintain or even restore damaged BBB in a number of neurodegenerative disorders, now including HD. It is clear from these studies that there is convincing evidence to show that the vasculature in human HD is compromised and contributes in a major way to degeneration of the neurons through compromised functioning of the blood vessels in HD brains in brain perfusion as well as clearance of toxins. This also increases the possible development of inflammatory conditions in the extracellular spaces of the brain, compromising the functioning of neurons that are already susceptible through aberrant functioning induced by the mutant gene they carry. What is not yet clear is at what stage of the disease the blood vessels are compromised, and much more work needs to be directed at this question, using both imaging biomarkers of BBB function and blood biomarkers such as S100b10,11 in living patients. Furthermore, these important studies of Hsaio et al1 and Drouin-Ouellet et al2 now need to be followed up with more detailed studies of how closely the neuropathology in the brain is associated with disrupted neurovasculature, particularly given the highly heterogeneous nature of neuropathological changes seen throughout the brain, which correlates with the heterogeneity of HD symptom profile.12–15 These findings raise a critical question regarding the relation of vascular changes to neurodegeneration in HD. If the regional heterogeneous neurodegeneration in the striatum and cerebral cortex in HD is also matched by a corresponding regional heterogeneity in vascular changes, then it follows that the vascular changes may possibly predate and predispose to the pattern of neurodegeneration in HD. Thus, if the disruption to the vasculature occurs early on in HD, this may prove to be an important target for drug development to treat this devastating genetic neurological disorder in the very early stages of the disease before the onset of substantial brain cell death.
Potential Conflicts of Interest Nothing to report.
Henry J. Waldvogel, PhD, Mike Dragunow, PhD, Richard L. M. Faull, MD, PhD, DSc Centre for Brain Research Faculty of Medical and Health Sciences University of Auckland Auckland, New Zealand
References 1.
Hsiao HY, Chen YC, Huang CH, et al. Aberrant astrocytes impair vascular reactivity in Huntington disease. Ann Neurol 2015 (current issue).
2.
Drouin-Ouellet J, Sawiak SJ, Cisbani G, et al. Cerebrovascular and blood-brain barrier impairments in Huntington disease: potential implications for its pathophysiology. Ann Neurol 2015 (current issue).
3.
Zlokovic BV. Neurovascular pathways to neurodegeneration in Alzheimer’s disease and other disorders. Nat Rev Neurosci 2011;12: 723–738.
4.
Guan J, Pavlovic D, Dalkie N, et al. Vascular degeneration in Parkinson’s disease. Brain Pathol 2013;23:154–164.
5.
Jellinger KA. Prevalence and impact of cerebrovascular lesions in Alzheimer and Lewy body diseases. Neurodegener Dis 2010;7: 112–115.
6.
Lin CY, Hsu YH, Lin MH, et al. Neurovascular abnormalities in humans and mice with Huntington’s disease. Exp Neurol 2013; 250:20–30.
7.
Armulik A, Genove G, Mae M, et al. Pericytes regulate the bloodbrain barrier. Nature 2010;468:557–561.
8.
Halliday MR, Rege SV, Ma Q, et al. Accelerated pericyte degeneration and blood-brain barrier breakdown in apolipoprotein E4 carriers with Alzheimer’s disease. J Cereb Blood Flow Metab 2015; doi:10.1038/jcbfm.2015.44.
9.
Winkler EA, Sengillo JD, Sullivan JS, et al. Blood-spinal cord barrier breakdown and pericyte reductions in amyotrophic lateral sclerosis. Acta Neuropathol 2013;125:111–120.
10.
Marchi N, Cavaglia M, Fazio V, et al. Peripheral markers of bloodbrain barrier damage. Clin Chim Acta 2004;342:1–12.
11.
Montagne A, Barnes SR, Sweeney MD, et al. Blood-brain barrier breakdown in the aging human hippocampus. Neuron 2015;85: 296–302.
12.
Nana AL, Kim EH, Thu DC, et al. Widespread heterogeneous neuronal loss across the cerebral cortex in Huntington’s disease. J Huntingtons Dis 2014;3:45–64.
13.
Thu DC, Oorschot DE, Tippett LJ, et al. Cell loss in the motor and cingulate cortex correlates with symptomatology in Huntington’s disease. Brain 2010;133(pt 4):1094–1110.
14.
Tippett LJ, Waldvogel HJ, Thomas SJ, et al. Striosomes and mood dysfunction in Huntington’s disease. Brain 2007;130(pt 1):206–221.
15.
Waldvogel HJ, Kim EH, Tippett LJ, et al. The neuropathology of Huntington’s disease. Curr Top Behav Neurosci 2015;22:33–80.
DOI: 10.1002/ana.24445
August 2015
159
Disrupted Vasculature and Blood–Brain Barrier in Huntington Disease
H
untington disease (HD) is characterized by neuronal loss in the caudate nucleus and putamen as well as heterogeneous patterns of neurodegeneration throughout various regions of the cerebral cortex and in many other regions of the brain. Two new studies by Hsaio et al1 and Drouin-Ouellet et al2 report that the cerebral vascular system is also highly disrupted in HD, adding a new dimension to the horizons of pathological mechanisms operating in this tragic monogenetic disease. The methods used to detect these changes included scanning HD-affected subjects using magnetic resonance imaging (MRI) as well as morphological studies of human postmortem brain tissue and brain tissue of animal models of the disease. The disruption of the vasculature and blood–brain barrier (BBB) has been strongly implicated in other neurodegenerative diseases such as Alzheimer disease3 and Parkinson disease.4 In the case of Alzheimer disease, there is deposition of b-amyloid in arteries resulting in cerebral amyloid angiopathy, which is present in >80% of Alzheimer disease cases.5 This causes dysfunction of vascular integrity, leading to leakage through the BBB allowing normally highly regulated molecules to cross into the brain, which can cause inflammatory cascades and release neurotoxins into the intracellular spaces of the brain. In addition, loss of vascular integrity leads to loss of blood flow and subsequent hypoxic regions throughout the brain as well as a lack of clearance of neurodegenerative products. The recognition of these types of vascular changes is only now becoming evident in HD. A previous study6 reported increased blood vessel density but not blood vessel size in the R6/2 mouse model of HD, as well as increased vessel density in the caudate nucleus of postmortem HD human brain. The results published in this new report by Hsiao et al1 further advance the field by showing marked changes in MRI that are measured as vascular reactivity in different mouse models of HD. The results of these knockin models indicate that the vascular density changes occur when mutant protein is expressed in both neurons and astrocytes rather than neurons alone. Additionally, there is altered cerebral blood flow in these mouse models that mimics the changes observed in human brains. One of the mechanisms highlighted in these studies is that an inflammatory response may lead to vessel changes. This was proposed by observing higher than normal levels of vascular endothelial growth factor A in astrocytes in brains C 2015 American Neurological Association 158 V
of mouse models and in human HD brain and that this promotes proliferation of new blood vessels, leading to increased vessel density. This inflammatory response was enhanced through an IjB kinase-nuclear factor jB– dependent pathway, and it may be through this mechanism that proliferation of endothelial cells is triggered; also, this importantly leads to the reduced coverage of pericytes. The second study by Drouin-Ouellet et al2 revealed that in HD patients there is a significant increase in cerebral blood flow in cortical gray matter but not in caudate or putamen, although the results did not yield differences in blood vessel densities in these scans. Following on from these imaging studies, morphometric studies of human postmortem brain found that in the putamen of HD patients there was an increased density of small blood vessels with a larger proportion of small blood vessels compared to medium-sized blood vessels; when correlated with putaminal surface area, this suggested that putaminal atrophy is accompanied by a reduction in blood vessel size. These changes in human brain also correlated with changes observed in the striatum of the R6/2 mouse, where the density of blood vessels was increased and there was a reduction in blood vessel diameter. Investigations at the cellular level in the human HD brain found inclusions in about 3 to 7% of cortical blood vessels that were apparent in all compartments of the neurovascular unit, including the basement membrane, endothelial cells, perivascular macrophages, and smooth muscle actin (SMA)-positive pericytes. In the R6/2 mouse, inclusions were found in the basal membrane, a-SMA positive pericytes or smooth muscle cells, and endothelial cells. There was also evidence of leakage of the BBB in the putamen of HD brains and in the striatum of the R6/2 mice. There was a decrease in proteins associated with tight junctions and an increase in vesicles associated with transcytosis, which may together increase BBB permeability. Recent studies suggest that pericytes regulate the maintenance of the BBB.7 In Alzheimer disease and in motor neuron disease, reduced capillary pericyte coverage has been associated with BBB leakage.8,9 Although Drouin-Ouellet et al2 did not find reduced numbers of pericytes in R6/2 mice using transmission electron microscopy, in the study by Hsaio et al pericyte coverage was found to be reduced in both the human HD brain
Waldvogel et al.: Disrupted Vasculature in HD
and in R6/2 mice using desmin or platelet-derived growth factor receptor b to label pericytes. Hsaio et al further showed that astrocytes may be responsible for pericyte loss in HD. Thus, further study of pericytes in HD and its models is vital in understanding vascular dysfunction in this disease. Because pericytes are critically involved in maintaining the BBB, compounds that augment their proliferation and/or viability may prove useful in helping to maintain or even restore damaged BBB in a number of neurodegenerative disorders, now including HD. It is clear from these studies that there is convincing evidence to show that the vasculature in human HD is compromised and contributes in a major way to degeneration of the neurons through compromised functioning of the blood vessels in HD brains in brain perfusion as well as clearance of toxins. This also increases the possible development of inflammatory conditions in the extracellular spaces of the brain, compromising the functioning of neurons that are already susceptible through aberrant functioning induced by the mutant gene they carry. What is not yet clear is at what stage of the disease the blood vessels are compromised, and much more work needs to be directed at this question, using both imaging biomarkers of BBB function and blood biomarkers such as S100b10,11 in living patients. Furthermore, these important studies of Hsaio et al1 and Drouin-Ouellet et al2 now need to be followed up with more detailed studies of how closely the neuropathology in the brain is associated with disrupted neurovasculature, particularly given the highly heterogeneous nature of neuropathological changes seen throughout the brain, which correlates with the heterogeneity of HD symptom profile.12–15 These findings raise a critical question regarding the relation of vascular changes to neurodegeneration in HD. If the regional heterogeneous neurodegeneration in the striatum and cerebral cortex in HD is also matched by a corresponding regional heterogeneity in vascular changes, then it follows that the vascular changes may possibly predate and predispose to the pattern of neurodegeneration in HD. Thus, if the disruption to the vasculature occurs early on in HD, this may prove to be an important target for drug development to treat this devastating genetic neurological disorder in the very early stages of the disease before the onset of substantial brain cell death.
Potential Conflicts of Interest Nothing to report.
Henry J. Waldvogel, PhD, Mike Dragunow, PhD, Richard L. M. Faull, MD, PhD, DSc Centre for Brain Research Faculty of Medical and Health Sciences University of Auckland Auckland, New Zealand
References 1.
Hsiao HY, Chen YC, Huang CH, et al. Aberrant astrocytes impair vascular reactivity in Huntington disease. Ann Neurol 2015 (current issue).
2.
Drouin-Ouellet J, Sawiak SJ, Cisbani G, et al. Cerebrovascular and blood-brain barrier impairments in Huntington disease: potential implications for its pathophysiology. Ann Neurol 2015 (current issue).
3.
Zlokovic BV. Neurovascular pathways to neurodegeneration in Alzheimer’s disease and other disorders. Nat Rev Neurosci 2011;12: 723–738.
4.
Guan J, Pavlovic D, Dalkie N, et al. Vascular degeneration in Parkinson’s disease. Brain Pathol 2013;23:154–164.
5.
Jellinger KA. Prevalence and impact of cerebrovascular lesions in Alzheimer and Lewy body diseases. Neurodegener Dis 2010;7: 112–115.
6.
Lin CY, Hsu YH, Lin MH, et al. Neurovascular abnormalities in humans and mice with Huntington’s disease. Exp Neurol 2013; 250:20–30.
7.
Armulik A, Genove G, Mae M, et al. Pericytes regulate the bloodbrain barrier. Nature 2010;468:557–561.
8.
Halliday MR, Rege SV, Ma Q, et al. Accelerated pericyte degeneration and blood-brain barrier breakdown in apolipoprotein E4 carriers with Alzheimer’s disease. J Cereb Blood Flow Metab 2015; doi:10.1038/jcbfm.2015.44.
9.
Winkler EA, Sengillo JD, Sullivan JS, et al. Blood-spinal cord barrier breakdown and pericyte reductions in amyotrophic lateral sclerosis. Acta Neuropathol 2013;125:111–120.
10.
Marchi N, Cavaglia M, Fazio V, et al. Peripheral markers of bloodbrain barrier damage. Clin Chim Acta 2004;342:1–12.
11.
Montagne A, Barnes SR, Sweeney MD, et al. Blood-brain barrier breakdown in the aging human hippocampus. Neuron 2015;85: 296–302.
12.
Nana AL, Kim EH, Thu DC, et al. Widespread heterogeneous neuronal loss across the cerebral cortex in Huntington’s disease. J Huntingtons Dis 2014;3:45–64.
13.
Thu DC, Oorschot DE, Tippett LJ, et al. Cell loss in the motor and cingulate cortex correlates with symptomatology in Huntington’s disease. Brain 2010;133(pt 4):1094–1110.
14.
Tippett LJ, Waldvogel HJ, Thomas SJ, et al. Striosomes and mood dysfunction in Huntington’s disease. Brain 2007;130(pt 1):206–221.
15.
Waldvogel HJ, Kim EH, Tippett LJ, et al. The neuropathology of Huntington’s disease. Curr Top Behav Neurosci 2015;22:33–80.
DOI: 10.1002/ana.24445
August 2015
159
